Control of a linear motor

ABSTRACT

Disclosed is a linear electric motor having a fixed primary comprising a stator divided into a number of sections, including a translating secondary having an operative length longer than any two adjacent sections of the stator in the form of a reaction plate, and a connecting means for connecting only those sections of the stator that are at least partially covered by the reaction plate. The position of the reaction plate relative to the stator is determined by monitoring current in the active representative sections. Power is supplied to each stator section individually, with power supplied in a modulated manner to end active stator sections only partially covered by the reaction plate. A measurement of the current to the active representative section is used to control output voltage to all energized stator sections and is used to determine the change in position of the reaction plate.

THIS INVENTION relates to an electric linear motor and the controlthereof.

In accordance with the invention there is provided an linear motor whichincludes

a fixed primary that is divided into a number of sections;

a translating secondary that has an operative length that is longer thanany two adjacent sections of the stator; and

a connecting means for connecting only those sections of the primarythat are in the vicinity of the secondary to a power supply.

Further in accordance with the invention there is provided a method ofcontrolling a linear motor having a fixed primary that is divided into anumber of sections and has a translating secondary that has an operativelength that is longer than any two adjacent sections of the primary,which includes

determining which of the primary sections are in the vicinity of thesecondary; and

energising only those sections that are in the vicinity of thesecondary.

Still further in accordance with the invention there is provided acontrol arrangement for a linear motor having a fixed primary that isdivided into a number of sections and a translating secondary that hasan operative length that is longer than any two adjacent sections of theprimary; which includes

a connecting means for connecting only those sections of the primarythat are in the vicinity of the secondary to a power supply.

In particular, those sections of the primary that are covered by thesecondary may be connected to the power supply.

The term “covered”, in relation to a particular section of the primary,is to be understood to mean that the secondary is aligned with thatsection; and “covers” will have a comparable meaning. Further, a sectionwill be “fully covered” if the secondary extends beyond the ends thereofand will be “partially covered” if the secondary overlies only a partthereof.

It will be appreciated that the primary sections need not be physicallydivided separate sections. The segmentation merely needs to be in termsof electrical supply to the windings.

The primary may comprise a stator and the secondary a reaction plate.The operative portion of the reaction plate will then be used todetermine if a stator section is covered, either fully or partially.

Each stator section may be connectable independently to the powersupply.

For ease of convenience, those stator sections that interact with thereaction plate are referred to hereinafter as “active” sections.

The connecting means may include a number of switches, there being aswitch for each stator section, and a control means for controllingopening and closing of the switches. For a multi-phase stator winding,each “switch” may comprise several switches in order toconnect/disconnect all of the phases of each stator section to/from thepower supply.

Those stator sections that are fully covered may be, in use, energised.It will be appreciated that, as the reaction plate moves relatively tothe stator sections, that there will, at times, be end stator sectionsthat are only partially covered by the reaction plate, the degree ofcover decreasing at one end and increasing at the other end, in thedirection of travel of the reaction plate. Should these partiallycovered stator sections be considered active and hence energised, it isdesirable that the magnitude of the current flowing in these outerstator sections is comparable to that flowing in the other active statorsections, such that the force produced per unit length of the activestator sections is approximately constant. However, the inductance of,as well as the back EMF produced in, the partially covered end statorsections varies with coverage and is lower than that of the fullycovered stator sections. Thus, if these partially covered sections aretreated as fully covered active sections and are merely connected inparallel to the voltage supply, the resulting currents would be toohigh. The invention may then include modulating the voltage supply tothese partially covered stator sections by using the switch connectingthe stator section to the power supply to switch the stator section onand off the supply with a duty cycle related to the degree of coverageof the stator section. The control means may thus include a modulatingmeans.

The current flow through the stator sections may be used to determinethe position of the reaction plate and which of the stator sections are,in operation, active. A filtering means may be provided to filter outtransients in current flow measurement. Alternatively, or in addition,the inductance of the stator sections may be used to determine which ofthe stator sections are active, and thus the position of the reactionplate, since the inductance of each stator section is related to thedegree of its coverage by the reaction plate. Thus, the control meansmay include an inductance measuring means for measuring the inductanceof a selected set of stator sections. The selected set may be a singlesection or a plurality of sections. On start-up, an initial positionreference point may be established by means of a binary or other searchprocedure. For a binary search, the combined inductance of all of thestator sections comprising one half of the total length of stator can bemeasured, followed by measuring the inductance of the other half of thetotal length of stator. It can hence be determined that the reactionplate is within the half having an inductance that has been influencedby the higher inductances of the active stator sections. This process isrepeated recursively (halving the number of stator sections used for thesearch at each iteration) until the reaction plate position is known towithin a pole pair. One approach to measuring inductance is to measurethe time taken for current to decay within the windings after brieflyenergising them. The winding inductance is proportional to the timeconstant of the decay (for a constant resistance). Such excitation maybe such that significant force transients do not result in the motor. Itwill be appreciated that the resistance of the stator windings willdepend on the temperature thereof. Thus, a temperature determining meansmay be provided for determining the temperature of at least one statorsection.

The stator sections may all have the same length. Thus the length of thestator sections is at most a half that of the reaction plate. It will beappreciated that control of the motor will depend on the size of thestator sections compared to the operative length of the reaction plate.The optimum stator section length will vary depending on a number oftrade-offs. For example, the shorter the length of each stator section,the greater the number of stator section switches required, yet theshorter the length of inactive reaction plate.

The stator sections may be categorised into two or more groups. In oneembodiment they may be categorised into three groups, being first andsecond groups of “representative” stator sections and a third group of“ordinary” stator sections. The first and second representative statorsections may alternate, such that there is a sequence of first, second,first, second, etc representative stator sections, with ordinary statorsections between them. The representative groups may comprise individualstator sections. There may be one or more ordinary stator sectionbetween each first and second representative stator section. The numberof ordinary stator sections between a first representative statorsection and the next second representative stator section may be thesame as that between a second representative stator section and the nextfirst representative stator section. Thus the distance between the startof a first representative stator section and the end of the next secondrepresentative stator section, including the ordinary stator sectionsbetween, may be the same as that between a second representative statorsection and the end of the next first representative stator section. Thecurrent flowing into a representative stator section may be used toinfer information regarding the entire interaction of the reaction platewith the stator. Hence, at any particular point in time, the currentflowing in a single active representative stator section can be used bya controller to output an appropriate switching pattern for the invertersupplying all of the active stator sections. Further, if an independentmeasurement of the reaction plate's position relative to the stator isnot being used (i.e. sensorless control), the position can be inferredfrom the position determined to within a pole-pair at start-up and anongoing estimate of the electrical angle between the reaction plate fluxand the in use representative stator section winding, as calculated bythe controller.

Further, several reaction plates and several stators may be mountedalongside each other that are mechanically and electrically coupled soas to act as a single linear motor.

The invention is now described, by way of an example, with reference tothe accompanying diagrammatic drawing, which shows a linear motor inaccordance with the invention.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a configuration of a linear electric motor in accordancewith the claimed subject matter.

With reference to the drawings, a linear electric motor in accordancewith the invention is designated generally by reference numeral 10. Themotor 10 has a fixed stator 12 and a translating reaction plate 14. Themotor 10 further has a control arrangement.

The stator 12 is segmented into constant length sections 18 along thelength of travel of the reaction plate 14, where each section 18 can beseparately fed from an electrical power source. This segmentation may beachieved by creating physically separate stator modules or could merelyentail the provision of connections to sections of the stator winding.

The stator sections 18 are categorised into three groups 18.1, 18.2 and18.3 along the length of travel of the reaction plate 14. The group 18.1is a first group of representative stator sections; the group 18.2 is asecond group of representative stator sections; and the group 18.3 is agroup of ordinary sections. A suitable number of ordinary statorsections 18.3 are provided between each first representative section18.1 and the next adjacent second representative section 18.2 andbetween each second representative section 18.2 and the next adjacentfirst representative section 18.1. It is to be noted that there are nophysical differences between the different groups of stator sections.Each stator section 18.1, 18.2 and 18.3 has a correspondingelectronically controlled switch 20 that connects it to the output of aninverter 22, which is supplied from a suitable power source via terminal24. If the linear motor 10 is a multi-phase machine, each switch 20represents multiple switches that operate concurrently (or with a slightdelay to allow for zero-crossing switching). Furthermore, there may beswitches (not shown in the drawing) that act to short the statorwindings (possibly through a braking resistor) when there is a powerfailure to enable dynamic braking. The switches 20 may beelectromechanical relays or electronic switches that use siliconcontrolled rectifiers (SCRs), insulated gate bipolar transistors (IGBTs)or any other electronic switching technology.

As seen in the drawing, the reaction plate 14 has a length that is equalto the distance from the beginning of a first representative statorsection 18.1 to the end of the ordinary stator section 18.3 immediatelyfollowing the next second representative stator section. 18.2. Thus, asseen in the drawing, with the reaction plate 14 being positionedrelative to the stator 12 as shown, the stator sections 18.2.1, 18.3.1and 18.1.1 are fully covered by the reaction plate 14. The statorsection 18.3.2 (ie the ordinary stator section immediately preceding thestator section 18.2.1) is also fully covered. With the basic controlstrategy of the invention, only these stator sections that are fullycovered by the reaction plate 14 are switched on to the output of theinverter 22. It will be understood that those sections of the statorthat are fully covered by the reaction plate are those stator sectionsthat are able to fully interact with the reaction plate 14.

Three sets of current sensors 26.1, 26.2 and 26.3 are used, as shown inthe drawing, to measure the current flowing into each of the threegroups of active stator sections, which are fed off different supplycables 28.1, 28.2 and 28.3. A controller 30, sets the switching state ofthe inverter 22 and controls operation of the switches 20, in accordancewith the relative position of the reaction plate 34 as determined by aposition determining module 38 incorporated in the controller 30. Thecontroller 30 makes use of the measurement of current flowing intoeither the first representative stator section 18.1 or the currentflowing into the second representative stator section 18.2 to determinethe position of the reaction plate 34. Signals provided by the sensors26 are supplied to a current measuring module 40 after being filtered bya filter 42 to remove transients. The current values provided by thecurrent measuring module are supplied to the position determining module38. Note that the functionality represented by controller 30 may beimplemented as part of a distributed or hierarchical control system. Themultiplexing between the measurements of current flowing into therepresentative stator sections 18.1 or 18.2, illustrated by switch 32,may be performed either in hardware or software. The measurement ofcurrent flowing into the ordinary stator sections 18.3 may be used forthe detection of fault conditions or for determining the initialposition of the reaction plate 14, as described below. The magnitude ofthe output voltage of the inverter 22 is common to all active statorsections and a measurement of this voltage may also be utilised by thecontroller 30. Since both active representative stator sections 18.1 and18.2 are fully covered by the reaction plate 14, they are electricallyindistinguishable from stators of rotary machines. This is significantin that a control technique originally designed for rotary machines andwhich may use a measurement of stator current can be used to control thelinear motor with the described stator switching strategy. Furthermore,measurements of the temperature of the active stator sections or anaverage temperature measured over several stator sections may be used bythe controller 30 to compensate for variation in each stator section'swinding resistance. A temperature sensor 46 and temperature measuringmodule 48 are indicated.

A description of the typical sequence of events that must occur as thereaction plate 14 moves up the stator 12, in the direction indicated byarrows 34, from the position shown in the drawing, is as follow. Oncethe reaction plate 14 fully covers the first representative statorsection 18.1.1, this stator section is connected to the output of theinverter by closing switch 20.1. At the same time, ordinary statorsection 18.3.2 is disconnected from the supply by opening switch 20.2,as it is about to no longer be fully covered by the reaction plate 14.At this point, the current flowing into the second representative statorsection 18.2.1 is used to perform control of the motor. It is only oncethe transients in the current flowing into the newly switched-in firstrepresentative stator section 18.1.1 have died down sufficiently thatthe multiplexer 32 selects the current flowing into the firstrepresentative stator section 18.1.1 as the control variable. However,this transition must take place whilst second representative statorsection 18.2.1 is still fully covered by the reaction plate 14. Thus, atthe time of the changeover, the currents flowing into the two fullycovered representative stator sections 18.1.1 and 18.2.1 are similarenough for the controller not to detect a significant disturbance.Furthermore, filtering of the measurement used by the controller may beperformed to reduce transients that may result from the two currentmeasurements not being precisely equal at the switching time. Theswitching sequence continues as the reaction plate 14 moves up thestator 12 with the multiplexer 32 selecting alternately between themeasurements of current flowing into the first representative statorsections 18.1 and the second representative stator sections 18.2.

An extension to the described basic switching strategy, for improvedefficiency at the expense of complexity, is to supply the two outerstator sections that are only partially covered by the reaction plate 14in addition to all the stator sections that are fully covered by thereaction plate. However, the voltage supplied to the outer statorsections must be modulated using the stator switches 20 to reduce themagnitude of the corresponding currents. This is because the inductanceof a partially covered stator section is lower than that of a fullycovered section and there is less induced back EMF. The control of thecurrents in the outer stator sections is achieved by switching (fasterthan the rated frequency of the motor) each of the two sections on andoff, with a duty cycle that results in a similar magnitude of current asis flowing in the fully covered stator sections, by a modulator 50.

If sensorless control is to be achieved, there needs to be a way offinding the initial position of the reaction plate 14 to within apole-pair. Thereafter, the “angular position estimate” from a suitablemotor control algorithm running in the controller 30 can be used to keeptrack of the position of the reaction plate 14. Fortunately, a binarysearch can be used to find this initial position because the inductanceof each section 18 is dependent on how much of it is covered by thereaction plate 14. The inductance of a portion of the winding can beeffectively measured by timing the current decay after the relevantsection of stator is briefly energised, by an inductance measuringmodule 44. The time-constant for the decay is directly proportional tothe inductance (for a constant resistance). The first step to findingthe position of the reaction plate 14 is to measure the inductance ofall the alternate representative stator groups in one half of the lengthof travel of the reaction plate 14, followed by the inductance of allthe alternate representative stator groups in the other half. Thereaction plate 14 is then known to be in the half of the stator 12 withthe higher inductance. The process is repeated in the half that containsthe reaction plate 14 until its position is obtained to within arepresentative stator section. Once the position of the reaction plate14 is known to within a representative stator section, the inductancesof the stator sections above the particular representative section aremeasured, one-by-one, until the edge of the reaction plate 14 is foundto within a stator section. Finally, the edge of the reaction plate 14is found to within a pole pair, since the inductance of the statorsection facing the edge is related to the number of poles covered by thereaction plate. The inductance of the stator section facing the otheredge of the reaction plate can be used as a check. Clearly, the fullsearch procedure is not necessary every time the controller is enabledbecause the controller can first check if the reaction plate has movedfrom its previous location by starting to search at this location.

The optimum length for the portion of the reaction plate 14 thatinteracts with the stator 12 is as shown in the drawing but the controlstrategy still applies for a reaction plate 14 that is up to a fullstator section length shorter or longer than this.

1. A linear motor which includes a fixed primary that is divided into anumber of sections, the sections being categorized into three groups,being a first group of representative sections, a second group ofrepresentative sections and a third group of ordinary sections; atranslating secondary that has an operative length that is longer thanany two adjacent sections of the primary; and a connecting means forconnecting only those sections of the primary that are covered by thesecondary to a power supply; the connecting means including anindependently operable switch for each section and a control means forcontrolling opening and closing of the switches; the control meanshaving a position determining means for determining which of thesections are covered by the secondary; and the control means having acurrent measuring means for measuring the current in the first andsecond representative sections with the position determining means beingresponsive thereto, in which the first and second representativesections alternate with at least one ordinary section from the thirdcroup between them; and the number of ordinary sections between a firstrepresentative section and the next second representative section is thesame as that between that second representative section and the nextfirst representative section.
 2. A linear motor as claimed in claim 1,in which the primary comprises a stator.
 3. A linear motor as claimed inclaim 1, in which the secondary comprises a reaction plate.
 4. A linearmotor as claimed in claim 3, in which the primary sections all have thesame length.
 5. A linear motor as claimed in claim 3, in which thelength of the reaction plate is equal to the distance from the beginningof a first representative section to the end of the ordinary sectionimmediately following the next second representative section.
 6. Alinear motor as claimed in claim 1, in which the current measuring meansincludes a filtering means for filtering out transients in currentmeasured thereby.
 7. A linear motor as claimed in claim 1, whichincludes a temperature determining means for determining the temperatureof at least one of the primary sections, the position determining meansbeing responsive thereto.
 8. A linear motor as claimed in claim 1, inwhich the control means includes a modulating means for modulatingoperation of the switches of those primary sections that are onlypartially covered by the secondary.
 9. A linear motor as claimed inclaim 1, in which the control means includes an inductance measuringmeans for measuring the inductance of a selected set of primarysections.
 10. A method of controlling a linear motor having a fixedprimary that is divided into a number of sections, the sections beingcategorized into three groups, being a first group of representativesections, a second group of representative sections and a third group ofordinary sections, and having a translating secondary that has anoperative length that is longer than any two adjacent sections of theprimary and with the first and second representative sectionsalternating with at least one ordinary section from the third groupbetween them and with the number of ordinary sections between a firstrepresentative section and the next second representative section beingthe same as that between that second representative section and the nextfirst representative section, which includes determining which of theprimary sections are covered by the secondary; and energizing only thosesections that are covered by the secondary; the primary sections thatare covered by the secondary being determined by measuring the currentin the first and second representative sections.
 11. A method ofcontrolling a linear motor as claimed in claim 10, in which the primarycomprises a stator.
 12. A method of controlling a linear motor asclaimed in claim 10, in which the secondary comprises a reaction plate.13. A method of controlling a linear motor as claimed in claim 10, inwhich the primary sections all have the same length.
 14. A method ofcontrolling a linear motor as claimed in claim 10, in which the lengthof the secondary is equal to the distance from the beginning of a firstrepresentative section to the end of the ordinary section immediatelyfollowing the next second representative section.
 15. A method ofcontrolling a linear motor as claimed in claim 10, which includesfiltering out transients in the measured current.
 16. A method ofcontrolling a linear motor as claimed in claim 10, which includesdetermining the temperature of at least one of the primary sections. 17.A method of controlling a linear motor as claimed in claim 10, whichincludes modulating the supply of power to those primary sections thatare only partially covered by the secondary.
 18. A method of controllinga linear motor as claimed in claim 10, which includes measuring theinductance of a selected set of primary sections in order to determinethe position of the secondary.
 19. A control arrangement for a linearmotor having a fixed primary that is divided into a number of sections,the sections being categorized into three groups, being a first group ofrepresentative sections, a second group of representative sections and athird group of ordinary sections, in which the first and secondrepresentative sections alternate with at least one ordinary sectionfrom the third group between them and in which the number of ordinarysections between a first representative section and the next secondrepresentative section being the same as that between that secondrepresentative section and the next first representative section, andhaving a translating secondary that has an operative length that islonger than any two adjacent sections of the primary, which includes aconnecting means for connecting only those sections of the primary thatare covered by the secondary to a power supply, the connecting meansincluding an independently operable switch for each section and acontrol means for controlling opening and closing of the switches; thecontrol means having a position determining means for determining whichof the sections are covered by the secondary and having a currentmeasuring means for measuring the current in the first and secondrepresentative sections, the position determining means being responsiveto the current measuring means.
 20. A control arrangement as claimed inclaim 19, in which the current measuring means includes a filteringmeans for filtering out transients in current measured thereby.
 21. Acontrol arrangement as claimed in claim 19, which includes a temperaturedetermining means for determining the temperature of at least one of thesections, the position determining means being responsive thereto.
 22. Acontrol arrangement as claimed in claim 19, in which the control meansincludes a modulating means for modulating operation of the switches ofthose sections that are only partially covered by the secondary.
 23. Acontrol arrangement as claimed in claim 19, in which the control meansincludes an inductance measuring means for measuring the inductance of aselected set of sections.
 24. A method of controlling a linear motorhaving a fixed primary that is divided into a number of sections, thesections being categorized into three groups, being a first group ofrepresentative sections, a second group of representative sections and athird group of ordinary sections, with the primary sections all havingthe same length, the first and second group of representative sectionsalternating with the same number of ordinary sections from the thirdgroup between them, and having a translating secondary that has anoperative length equal to the distance from the beginning of a firstrepresentative section to the end of the ordinary section immediatelyfollowing the next second representative section, which includesdetermining which of the primary sections are covered by the secondary;and energizing only those sections that are covered by the secondary;the primary sections that are covered by the secondary being determinedby measuring the current in the first and second representativesections.